124 The American Geologist. August, 189* 
found the remarkable series of deeply cut valleys which hold the Finger 
lakes. The altitudes of these lakes above the sea ranges from Cayuga 
and Seneca, which are the two lowest and largest, respectively at 378 
and 441 feet, to Canadice, the highest, at 1,099 feet. Cayuga and Seneca 
lakes each exceed thirty miles in length and have maximum widths of 
three miles, with maximum depths respectively 435 and G18 feet, in each 
case reaching below the level of the ocean. 
The main outlines of the contour of this region are believed to have 
been produced in pre-glacial times by subaerial erosion; and the chan- 
nels cut in the plateau by its streams flowing southward and northward 
appear to have become continuous by backward cutting at their head- 
waters, with only low cols separating them in the present Finger lake 
valleys. The narrowing of these valleys toward the southern ends of 
the lakes and at the present moraine-covered water parting leads to the 
conclusion that the streams then flowing where the Finger lakes now 
lie were tributary to the preglacial river whose valley now is occupied 
by lake Ontario. During the Glacial period ice erosion moulded the 
highlands in smoothly flowing slopes, crowned with long, gentle, drum- 
loidal swells, thinly enveloped by drift; but at the northern ends of the 
lakes a considerable depth of drift was deposited in the northward con- 
tinuation of their valleys, sufficient, probably, in the case of Cayuga 
lake to account for at least half of its depth, and for about two-fifths 
of the depth of Seneca lake. The residue of the depths of these lakes, 
besides the unknown but probably small thickness of their bottom 
sediments, is attributed to glacial excavation, aided to some degree by 
crust oscillation. Within recent time the erosion of small tributary 
streams has formed the beautiful Watkins and Havana glens and the 
Glen Ora and Hector falls on the borders of Seneca lake, the Eagle Cliff 
and Taghannock falls in and near Ithaca on Cayuga lake, and many 
other very picturesque ravines and waterfalls. 
The Origin of Igneous Rocks. By Joseph Paxton Iddixgs. Philo- 
sophical Society of Washington, Bull. vol. xii, pp. 89-214, June, 1892. 
Mr. Iddings has had ample opportunity in carefully studying several 
volcanic regions to collect much valuable data concerning the "consan- 
guinity" of igneous rocks, and in this paper he presents in detail his 
'■reasons for concluding that all the volcanic and other igneous rocks of 
any region are so intimately connected together by mineralogical and 
chemical relations that they must have originated from some single 
magma, whose composition may be different in different regions; and, 
further, that it is the chemical differentiation of this primary magma 
which gives rise to the various kinds of igneous rocks." 
The first part of the paper is devoted to a historical review of the 
ideas held by different geologists as to the nature and origin of igneous 
rocks. Beginning with Scrope, to whom we are indebted for the first 
modern treatise on volcanoes, the contributions of the more important 
authors are mentioned, chief among whom are Darwin, Dana. Bunsen, 
Von Walterhausen, Von Richtofen, King, Dutton, Rosenbusch and 
Brogger. In the next part of the paper the mineralogical, chemical 
