of Crystalline Combinations. 423 
OBLONG-PYRAMIDAL SYSTEM. 
Relations of Edges, &c. 
The form is a right pyramid with a rectangular base. 
A horizontal prism with vertical ends. 
E. C. parallel to terminal edges of the finite prism. 
A right rectangular prism, the section of which is an oblong. 
An example of this application in the Rhombohedral System is given 
in the Edinburgh Journal of Science for January, 1827. The following 
examples may shew its use in the Square-Pyramidal and Oblong-Pyramidal 
systems. 
Fig. 4, (Mohs I. 67, 68.) belongs to the Square-Pyramidal system. 
The faces a alone would form some pyramid pQ; the faces f, a pyramid 
p’Q; the faces b alone, some pyramid in a diagonal position p'Qr; the 
faces c, an eight-sided pyramid gQn; the faces d and e, prisms ©Qm, 
and »Qm’. Hence, the general designation of the form is 
PQ, p'Qr, Qn, ~Qm, ~Qm’, p’Q 
a b c d e if 
Let a belong to the fundamental pyramid: therefore, pQ =Q. The 
combination a, b agrees with p, 1, Class I, No. 4. The combination », e, 
would have horizontal edges; therefore, in the same manner, 
oQm’ = oQr. 
Vol. 11. Part IL. 3I 
