COLLEGNO ON THE LOMBARDIC ALPS. 117 



extends along the whole district, and may be seen in the suburbs 

 of Como dipping S. 20° E., and covered up with diluvium. 



(4.) The bed next succeeding is a marly limestone of a brick-red 

 colour, deposited in very even layers, four or five inches thick, and 

 remarkably uniform in its colour and general appearance, more 

 especially in the lower part. Notwithstanding this, the bed in 

 question occasionally passes into a mere calcareous marl containing 

 silex, which is, however, sometimes fossiliferous, and in this way 

 is determined to be of the age of the inferior oolite. It is perfectly 

 conformable to the underlying grey limestone, and passes by in- 

 sensible gradations into the overlying bed, locally called majolica. 

 This is strikingly seen on the road between Solzago and Ponzate. 



(5.) The bed which forms the upper part of the oolitic series, as 

 exhibited in the Lombardic Alps, has been distinguished by con- 

 tinental geologists by the Italian names majolica, scaglia, &c. It 

 is a white compact limestone, exhibiting conchoidal fracture, and 

 often full of large cavities partly filled up with crystalline car- 

 bonate of lime. This rock is often traversed by very narrow 

 blackish-coloured veins, and is marked throughout its entire thick- 

 ness by the occasional presence of silex. The brilliant whiteness 

 of this limestone is such that it can usually be seen from a great 

 distance ; but it is sometimes coloured, and occasionally converted 

 into dolomite. It has been often mistaken for chalk. 



Of these beds, the red marly limestone (4.) contains in some 

 places a number of remains of Ammonites ; and the bed thus cha- 

 racterised is very easily recognised in the Italian Alps, being found 

 at several points in the Apennines of Tuscany and in the Papal 

 States ; so that it appears to mark throughout Italy a geological 

 horizon of which it is important to fix the exact date. M. Ale. 

 d'Orbigny has recognised the following Ammonites from one locality 

 in which this bed appears ; and it will be seen from this list how 

 low down in the oolitic series it must be placed: — Ammonites 

 heterophyllus, Sow. A. elegans, Sow. A. jibulatus, Sow. A. 

 Walcoti, Sow. A. insignis, Zieten. A. radians, Schlott. A. 

 scipionianus, D'Orb. A. thouarensis, D'Orb. ; and A. comejisis, 

 described by M. Von Buch, is from the same locality. 



2. Cretaceous Rocks. 



There are four subdivisions of the cretaceous system developed 

 on the south of the Alps, some of them agreeing with beds pro- 

 bably contemporaneous in the south of France, and others almost 

 peculiar to the district. They are thus arranged in order of 

 superposition : 



4. Variegated red and blue marls. 



8. Nummulitic limestone. 



2. Sandstone, more or less argillaceous, with numerous impressions of 

 fucoids. 



1. A conglomerate, sometimes used for millstones, and containing occa- 

 sionally remains of Hippurites. 



i 3 



