508 Scientific Intelligence. 



period. No evidence is presented or obtainable to show that this 

 barrier was the present Isthmian region, however. The pale 

 ontologic evidence indicates clearly the existence of an ephemeral 

 passage at the close of the Kocene period which was closed near 

 the end of the Oligocene, and there is no evidence of Miocene, 

 Pliocene, Pleistocene or recent connection in the Isthmian section. 



The conception, carrying out, and publication of this valuable 

 memoir is due to the munificence of Professor Alexander Agassiz. 

 We are informed that he has in hand the manuscript of another 

 and larger report by Mr. Hill upon the detailed geology of the 

 Island of Jamaica, a type study of the Antillean geology which 

 will deal further with the problems of the origin of the tropical 

 American lands. 



2. Geology and Mineral Resources of the Judith Mountains oj 

 Montana; by W. H. Weed and L. V. Pirsson. From the 

 Eighteenth Annual Report of the Director of the U. S. Geolog- 

 ical Survey, Washington, 1898. — This paper is a very interesting 

 discussion of the geological history and mineral resources of the 

 Judith Mountain region in Central Montana. The Judith 

 Mountains form an independent group of elevations of limited 

 extent, rising at the highest point 6386 feet above the sea, or 

 nearly 3000 feet above the surrounding plain. Like other similar 

 districts in the northwest, the isolated position of the group, sur- 

 rounded as it is by a broad expanse of Cretaceous rocks, has given 

 rise to many interesting geological problems, for the working out 

 of which the conditions are peculiarly favorable. Perhaps the 

 most interesting point brought out in the present memoir is the 

 laccolithic character of the igneous intrusions which make up a 

 large part of the mountains. This subject is very iully treated 

 by Pirsson, with numerous illustrations, and the author proposes 

 to discuss it in an early number of this Journal. The character 

 of the igneous rocks is given in the following summary : 



"The igneous rocks of the Judith Mountains are of acid-feld- 

 spathic character, and are very like those characteristic of other 

 laccolithic areas. They comprise granite-porphyry, syenite, 

 syenite-porphyry, and diorite-porphyry in the main masses, with 

 dikes and sheets of the variety of phonolite-porphyry called 

 tinguaite-porphyry. W hile the intrusion of the former rocks has 

 taken place according to well-known processes, it is believed that 

 the phonolite porphyry was formed by some process of differentia- 

 tion in the main masses and was injected into the sediments above 

 them by what may be called secondary intrusion. The granular- 

 ity of the rocks depends on their chemical composition and not 

 on the depth at which they have been intruded." 



As regards the sedimentary rocks, which are most extensively 

 and regularly developed in the western part, the characteristic 

 thick-bedded limestones of Carboniferous age' cover much the 

 larger part of the area. Between these and the Cretaceous of 

 the plains (Dakota and Benton) are parallel bands belonging to 

 the Lower Cretaceous (Kootanie) and to the Jurassic (shales, 



