48 The American Geohxjist. July, lsog 
Further on the author treats of the very interesting chain of voh;anoes 
running nearly due east and west. These volcanic craters lie about 20 
kilometres distant from the city of Mexico, on the southeast side. The 
highest of these volcanoes is the Sta. Catarina, with an altitude of 600 
meters above the plain, having a crater 110 meters deep. The mountain 
of Santiago, as is shown by its dome-shaped form surrounded by a par- 
tially destroyed crater, has evidently sent forth a current of viscous 
lava, which has broken up and destroyed the crater, thus forming a 
mamelon. In this chain of volcanoes are several intersecting craters, 
which show that the centre of eruption has moved slowly in an easterly 
direction, the intensity of these eruptions varying in degree. 
In the latter part of the book the author gives a description of the 
physiography of the mountain of Las Cruces, which lies on the west 
side of the valley of Mexico, and separates it from the valley of Toluca. 
This mountain is formed of extensive hills at its base and is crowned by 
high mountains scarped gently at the top. 
A description is given of the very extensive formation of the pumice 
tvifa which represents the first products of that Tertiary volcano. The 
pumice tufa is extensively used in the building of houses and other 
structures. 
The work conchide.s with a petrographical description of the lava 
beds of the volcanoe;; of Sta. Catarina, considering them as augite-ande- 
site vi^ith hipersthene very similar to those of the volcano Popocatepetl. 
Rocks from Sierra de las Cruces are trachites and trachi-andesites with 
pyroxene and hornblende. 
In Mexico the trachite is called CliiUica, and is considered the best 
material for constrviction. 
Professor Ordonez hopes in subsequent articles to continue the petro- 
graphical description of all the rocks in the valley of Mexico and to 
describe in general the geology of this interesting region of Mexico. 
On Certain Grciiuypliyrea, Modified by the Ineorporation of Gahhro- 
FragnienU, in Stratli (Skye). By Alfred Harker. (Quart. Jour. 
Geoi. Soc, no. 206, vol. 52, pt. 2, pp. ,320-330, pis. 13 and 11, May 1, 1896.) 
In mapping, for the Geological Survey of Great Britain, the district of 
Strath in the Isle of Skye, Mr. Harker has studied some granophyres 
which contain numerous inclusions of gabbro, and sometimes of basalt. 
At times the granophyre has taken up foreign material amounting to 
one-fourth of its bulk. The gabbro fragments are thought to have 
come from a deep source, and they have in part been dissolved in the 
acid magma. For this reason the granophyre is of a higher specific 
gravity and contains more of the fei-ro-magnesian minerals and more 
lime-soda feldspar than do the surrounding and similar granoj)hyres. 
Fragments of augite, which has the peculiar basal striation of the 
augite of the gabbro, are quite common, and some slides show both 
foreign augite and augite native to the granophyre. One section is 
figured which contains a large plate of foreign augite showing distinct 
ophitic relations to feldspar crystals. 
