I08 REVIEWS 



solidification so that their grain continues to increase clear to the 

 center, while later-formed constituents increase only for a shorter 

 distance, their grain thereafter remaining uniform. This porphyritic 

 type will be most obvious at the center of the igneous mass. 



Crystals, the conditions (temperature) of whose formation were 

 nearly half way between those obtaining initially in the igneous magma 

 and the country rock. Such crystals will be most conspicuously por- 

 phyritic at or near the margin. 



Finally there may be crystals, which like the staurolite of schists, 

 are formed by metamorphic actions, of secondary origin, and occur in 

 sediments, and only casually occur in igneous rocks. 



Attention is particularly called to the third and fourth classes, the 

 possibility of the existence of which has been almost overlooked, 

 though their possible existence may be readily inferred from inspection 

 of diagrams of the cooling of an intrusive. Certain field observations 

 render their actual existence probable. 



The Basal Conglomerate i?t Lehigh a?id Northampton Coimties, 

 Pennsylva7iia. By Frederick B. Peck, Easton, Pa. 



The term ''basal conglomerate," as here used, refers to that lowest 

 member of series of beds, belonging to the Cambrian, which lies 

 uncomformably upon the pre-Cambrian gneisses, but is conformable 

 with the overlying lower Cambrian dolomites. It occurs here, as else- 

 where in Pennsylvania and New Jersey, fringing the pre-Cambrian 

 areas. In eastern Northampton county it fails occasionally as a result 

 of faulting. It has a thickness varying from a few feet near Easton to 

 one hundred or possibly several hundred feet at Alburtis, twenty-four 

 miles southwest of Easton. 



Petrographically, it is quite variable. At times it is a coarse con- 

 glomerate, made up of quartz pebbles, an inch or two in diameter. 

 Frequently it is a medium to fine grained arkose, consisting of about 

 one part feldspar (orthoclase or microcline) to two or three parts 

 quartz, the former usually thoroughly kaolinized, the latter badly 

 crushed, and under the microscope exhibiting an undulating extinc- 

 tion. Other phases of it present a dense bluish or grayish quartzite. 

 It occasionally contains interstratified beds of a very fine-grained 

 argillaceous sandstone with numerous worm borings (scolithus), but 

 as yet no distinctly lower Cambrian fossils have been found. The 

 seemingly uppermost member is a highly ferruginous, almost jaspery 



