Returning to New Haven, he studied its geology, and made a chemical analysis of the meteorite that fell near Weston, Connecticut, publishing the first scientific account of any American meteorite.
He lectured publicly at New Haven in 1808 and came to discover many of the constituent elements of many minerals.
Some time around 1818, Ephraim Lane took some samples of rocks he found at an area called Saganawamps, now a part of the Old Mine Park Archeological Site in Trumbull, Connecticut, to Silliman for identification.
Silliman reported in his new American Journal of Science, a publication covering all the natural sciences but with an emphasis on geology, that he had identified tungsten, tellurium, topaz and fluorite in the rocks.
He played a major role in the discoveries of the first fossil fishes found in the United States.
 In 1837, the first (and at the time only) prismatic barite ore of tungsten in the United States was discovered at the mine.
The mineral sillimanite was named after Silliman in 1850.
Upon the founding of the Medical School, he also taught there as one of the founding faculty members.

In 1833 he discussed the relationship of Flood geology to the Genesis account, and also wrote about this topic in 1840.

Silliman was an early supporter of coeducation in the Ivy League.
 Although Yale wouldn't admit women as students until over 100 years later, he allowed young women into his lecture classes.
 His efforts convinced Frederick Barnard, later President of Columbia College, that women ought to be admitted as students. "
The elder Silliman, during the entire period of his distinguished career as a Professor of Chemistry, Geology and Mineralogy in Yale College, was accustomed every year to admit to his lecture-courses classes of young women from the schools of New Haven.
 In that institution the undersigned had an opportunity to observe, as a student, the effect of the practice, similar to that which he afterward created for himself in Alabama, as a teacher.
 The results in both instances, so far as they went, were good; and they went far enough to make it evident that if the presence of young women in college, instead of being occasional, should be constant, they would be better."

American historian David McCullough mentions in his book about early 19th century Americans in Paris that in 1825 Professor Benjamin Silliman while on a tour of Europe conferring with other scientists encountered his former Yale science student Samuel F. B. Morse in the Louvre.
McCullough also relates that Silliman would later become president of the college.

As professor emeritus, he delivered lectures at Yale on geology until 1855; Benjamin Silliman Sr had been the first person to use the process of fractional distillation, and, in 1854, Benjamin Silliman Jr became the first person to fractionate petroleum by distillation.

In 1864, Silliman noted oil seeps in the Ojai, California area.
 In 1866, this led to the start of oil exploration and development in the Ojai Basin.

Like his son-in-law James Dana, Silliman was a Christian.
In an address delivered before the Association of American Geologists he spoke in favor of old-earth creationism, stating:
It is already admitted by multitudes, that the chronology of the Scriptures is, in strictness, applied only to the history of our race, the sole moral beings whom God has placed in this World; while all that precedes man in the creation, is limited, in duration backwards, only by that beginning, whose date is known to no being but the infinite Creator, and which certainly precedes, by many ages, the creation of man.

— Silliman, (1842)
In the same line of thought, he posed arguments against atheism and materialism.
Returning to New Haven, he studied its geology, and made a chemical analysis of the meteorite that fell near Weston, Connecticut, publishing the first scientific account of any American meteorite. He lectured publicly at New Haven in 1808 and came to discover many of the constituent elements of many minerals. Some time around 1818, Ephraim Lane took some samples of rocks he found at an area called Saganawamps, now a part of the Old Mine Park Archeological Site in Trumbull, Connecticut, to Silliman for identification. Silliman reported in his new American Journal of Science, a publication covering all the natural sciences but with an emphasis on geology, that he had identified tungsten, tellurium, topaz and fluorite in the rocks. He played a major role in the discoveries of the first fossil fishes found in the United States.  In 1837, the first (and at the time only) prismatic barite ore of tungsten in the United States was discovered at the mine. The mineral sillimanite was named after Silliman in 1850. Upon the founding of the Medical School, he also taught there as one of the founding faculty members.
In 1833 he discussed the relationship of Flood geology to the Genesis account, and also wrote about this topic in 1840.
Silliman was an early supporter of coeducation in the Ivy League.  Although Yale wouldn't admit women as students until over 100 years later, he allowed young women into his lecture classes.  His efforts convinced Frederick Barnard, later President of Columbia College, that women ought to be admitted as students. "The elder Silliman, during the entire period of his distinguished career as a Professor of Chemistry, Geology and Mineralogy in Yale College, was accustomed every year to admit to his lecture-courses classes of young women from the schools of New Haven.  In that institution the undersigned had an opportunity to observe, as a student, the effect of the practice, similar to that which he afterward created for himself in Alabama, as a teacher.  The results in both instances, so far as they went, were good; and they went far enough to make it evident that if the presence of young women in college, instead of being occasional, should be constant, they would be better."
American historian David McCullough mentions in his book about early 19th century Americans in Paris that in 1825 Professor Benjamin Silliman while on a tour of Europe conferring with other scientists encountered his former Yale science student Samuel F. B. Morse in the Louvre. McCullough also relates that Silliman would later become president of the college.
As professor emeritus, he delivered lectures at Yale on geology until 1855; Benjamin Silliman Sr had been the first person to use the process of fractional distillation, and, in 1854, Benjamin Silliman Jr became the first person to fractionate petroleum by distillation.
In 1864, Silliman noted oil seeps in the Ojai, California area.  In 1866, this led to the start of oil exploration and development in the Ojai Basin.
Like his son-in-law James Dana, Silliman was a Christian. In an address delivered before the Association of American Geologists he spoke in favor of old-earth creationism, stating:
It is already admitted by multitudes, that the chronology of the Scriptures is, in strictness, applied only to the history of our race, the sole moral beings whom God has placed in this World; while all that precedes man in the creation, is limited, in duration backwards, only by that beginning, whose date is known to no being but the infinite Creator, and which certainly precedes, by many ages, the creation of man.
— Silliman, (1842)
In the same line of thought, he posed arguments against atheism and materialism.
Benjamin Silliman