146 MINERALOGY AND LITHOLOGY. 
to sub-divide their rocks into older and newer, and sharply to separate 
them in their nomenclature into two classes, according to their geologi- 
cal age, the tertiary being the turning point. This separation has been 
strongly opposed by some American and English geologists, and the ob- 
jection has been made good by proving that the distinctive characters 
are sometimes wanting where they should be found. Although the 
result of the discussion may be to eliminate the element of age from 
nomenclature, still the characters that distinguish old eruptive rocks 
from the newer ones are so very fundamental and so very general, that 
the value of these distinctions is recognized by every one. If, with this 
distinction in mind, one examines sections of our basic eruptive rocks, all 
the characters that are assigned to very old rocks are immediately recog- 
nized. They possess a crystalline structure that throws them into con- 
trast with younger rocks, and in their compositions and transformations 
they show all the effects of age. They do not come within the bounda- 
ries of the discussion before named, because geology and microscopy 
both would assign them to old formations. 
Of the basic eruptive rocks of this country, those that cut the Meso- 
zoic sandstone have been best studied. One vast series of dykes ex- 
tends up the Connecticut valley from the sound to the border of our 
state, and though they do not come within our boundaries, and hence 
are not within the limits of our descriptions, it is instructive to compare 
our rocks with a well defined American formation. We are indebted to 
Prof. J. D. Dana for most of our knowledge of these rocks, and Mr. E. S. 
Dana has examined them with the microscope. These rocks are essen- 
tially uniform in general appearance and in mineral composition wherever 
found, from Nova Scotia to Carolina. Some analyses made by myself 
indicate that their chemical composition is also almost invariable. Dia- 
base is the typical rock of the whole formation, varying only in the 
amount of hydration and alteration. Sometimes it is clear and undecom- 
posed, with scarcely a trace of hydrous minerals, and sometimes its con- 
stituents are all hydrated and decomposed, but in all cases it bears evi- 
dence of having been the same in original composition. These rocks 
were erupted after the accumulation of the Mesozoic red sandstones. 
Turning now from this grand uniform system to our old trap dykes, all 
this symmetry disappears. Not only do we find that there are varieties 
